Transformation of heavy metal fractions on soil urease and nitrate reductase activities in copper and selenium co-contaminated soil

• Cu tended to transform to organic matter-bound fractions, while Se mainly bound to Fe–Mn oxidates.
• IR (reduced partition index) can be used to describe the bioavailability of Cu and Se in soil.
• Soil urease and nitrate reductase activities exhibited a good correlation with IR value.
• Exchangeable Cu and organic matter-bound Se have direct effects on urease and nitrate activities.

This study aims to explore the effects of the distribution, transformation and bioavailability of different fractions of copper (Cu) and selenium (Se) in co-contaminated soils on soil enzymes, providing references for the phytoremediation of contaminated areas and agriculture environmental protection. Pot experiments and laboratory analysis were used to investigate the transformation and bioavailability of additional Cu and Se for pakchoi (Brassica chinensis) in co-contaminated soil. In the uncontaminated soil, Cu mainly existed in residual form, whereas Se was present in residual form and in elemental and organic-sulfide matter-bound form. In the contaminated soil, Cu mainly bound to Fe–Mn oxidates, whereas Se was in exchangeable and carbonates forms. After a month of pakchoi growth, Cu tended to transfer into organic matter-bound fractions, whereas Se tended to bound to Fe–Mn oxidates. The IR (reduced partition index) value of Cu decreased as the concentrations of Cu and Se gradually increased, whereas the IR value of Se decreased as the concentration of Se increased. The IR value before pakchoi planting and after it was harvested was not affected by the concentration of exogenous Cu. Soil urease and nitrate reductase activities were inhibited by Cu and Se pollution either individually or combined in different degrees, following the order nitrate reductase>urease. The significant correlation between the IR value and soil enzyme activities suggests that this value could be used to evaluate the bioavailability of heavy metals in soil. Path analysis showed that the variations in exchangeable Cu and organic-sulfide matter-bound and elemental Se had direct effects on the activities of the two enzymes, suggesting their high bioavailability. Therefore, the IR value and the transformation of metals in soil could be used as indicators in evaluating the bioavailability of heavy metals.